<!-- opacitu设置元素透明度，包括背景和内容 -->

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>opacity-透明度</title>
    <style>
        div {
            width: 500px;
            height: 500px;
            background-color: slategrey;
            background-image: url(../../Pictures/Desk2.jpg);
            /* background-repeat: no-repeat; */
            background-size: cover;
            
            /* opacity: 0; */
            /* opacity: 1; */
            opacity: 0.5;

        }
    </style>
</head>
<body>
    <div>
    </div>
</body>
</html>